Are Earth-like planets our best bet for finding extra-terrestrial life?

The expected planet and body sizes associated with an ordinary sentient species (right), relative to those of an ordinary sentient individual (left).
The expected planet and body sizes associated with an ordinary sentient species (right), relative to those of an ordinary sentient individual (left).
Research
(04/01/2016)

Our planet, the Earth, is not a  fair representation of other life-bearing planets, while mankind is not a fair reflection of intelligent species, concludes a study published in the Monthly Notices of the Royal Astronomical Society and authored by Fergus Simpson, researcher at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB, IEEC-UB).


 

 

The expected planet and body sizes associated with an ordinary sentient species (right), relative to those of an ordinary sentient individual (left).
The expected planet and body sizes associated with an ordinary sentient species (right), relative to those of an ordinary sentient individual (left).
Research
04/01/2016

Our planet, the Earth, is not a  fair representation of other life-bearing planets, while mankind is not a fair reflection of intelligent species, concludes a study published in the Monthly Notices of the Royal Astronomical Society and authored by Fergus Simpson, researcher at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB, IEEC-UB).


 

 

Using a statistical calculation, Simpson estimates that most sentient extra-terrestrial species exceed 300 kg, similar to the mass of an adult polar bear. This result is based solely on the premise that, on average, physically larger organisms possess smaller populations. Following a similar line of reasoning, Simpson also finds that the majority of planets which host life are smaller than the Earth. These results represent the first tangible insights into the nature of alien life.

“This result arises for the same reason that you are likely to be living in a country with over six million people, despite most countries possessing a lower population”, says the University of Barcelona researcher. “When it comes to groups —argues Simpson—, we should expect to be in a highly populated one, not an ordinary one. Larger planets are capable of hosting bigger populations, so this suggests that the Earth is abnormally large”. In much the same vein, smaller species are able to sustain larger populations, since each individual requires less energy and less space. “This indicates that we are likely to be the ants among intelligent species”, points out Simpson. In reaching this conclusion the researcher does not make any assumptions about the way evolution progressed on other planets, or whether there is any correlation between body size and intelligence.

The implications extend beyond the search for extra-terrestrial intelligence (SETI). Primitive forms of life are a pre-requisite for the development of sentient species, so our expectation of their habitats are also shifted. This could help decide which exoplanets are selected to look for chemical signatures of life in their atmospheres.

To calculate the expected size of a sentient alien, the author exploits the fact that on average, bigger organisms need more energy to survive. This striking trend, known as Kleiberʼs law, can be seen across all forms of life, from bacteria to whales. A fundamental  limitation on a speciesʼ population size is imposed by its energy requirement, in accordance with the laws of thermodynamics. This suggests that the tendency for larger species to possess lower population densities is a universal one, and not a unique feature of life on our planet.

Article reference:

F. Simpson. ʻThe Size Distribution of Inhabited Planets'. MNRAS Letters, Monthly Notices of the Royal Astronomical Society, Oxford University Press. Doi: 10.1093/mnrasl/slv170

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